1. Field of the Invention
The present invention relates to an organic electroluminescent device and its manufacturing method.
2. Description of the Related Art
An organic electroluminescent device is a light-emitting device which makes use of a principle that when an electric field is applied to the device, a fluoresent material emits light in response to a charge recombination of holes injected from an anode and electrons from a cathode. Such organic electroluminescent devices have been developed ever since C. W. Tang et al. published “Organic Electroluminescent Diodes”, Applied Physics Lett. 51(12), pp. 913-915, Sep. 21, 1987.
Tang et al. reported an electroluminecent device of a stacked structure using tris (8-quinolinol aluminum) in an emitting layer and a triphenyldiamine derivative in a hole-transporting layer. This stacked structure has advantages in that the injection efficiency of holes into the emitting layer can be improved, that electrons injected from a cathode can be blocked to increase the efficiency of exciton production from charge recombination, and that the excitons into the emitting layer can be confined. A multi-layered structure such as a double layered structure composed of a hole-injecting and transporting layer and an electron-transporting and emitting layer or a triple layered structure composed of a hole-injecting and transporting layer, an emitting layer and an electron-injecting and transporting layer is well known as an organic electroluminescent device. In order to increase the recombination efficiency of injected holes and electrons, various improvements have been introduced into the structure and fabrication process of the multi-layered structure.
A first prior art organic electroluminescent device is constructed by a transparent insulating substrate, striped lower electrodes made of indium tin oxide (ITO) formed on the transparent insulating substrate, an emitting layer formed on the lower electrodes, and striped upper electrodes formed on the emitting layer. In this case, the upper electrodes are arranged along a first direction, and the lower electrodes are arranged along a second direction normal to the first direction. As occasion demands, a hole-transporting layer is interposed between the lower electrodes and the emitting layer, and an electron-transporting layer is interposed between the emitting layer and the upper electrodes, to enhance the injection efficiency of carriers such as holes and electrons into the emitting layer. Note that the emitting layer, the hole-transporting layer and the electron-transporting layer are made of organic materials.
In the above-described first prior art organic electroluminescent device, however, since there are steps at the edges of the lower electrodes, the portions of the organic layers such as the hole-transporting layer, the emitting layer and the electron-transporting layer in proximity to the steps of the lower electrodes become thinner. Particularly, since the emitting layer is very thin, i.e., about 100 nm thick, when the portions of the emitting layer in proximity to the steps of the lower electrodes become thinner, a short-circuit would occur between the lower electrodes and the upper electrodes through the thinner portions of the emitting layer to damage the organic electroluminescent device.
A second prior art organic electroluminescent device includes an insulating pattern layer having elements filled between the lower electrodes to substantially make the edges of the lower electrodes smooth, thus avoiding a short-circuit between the lower electrodes and the upper electrodes through the organic layers including the emitting layer (see: JP-A-3-250583, JP-A-3-274694 & JP-A-4-51494).
In the above-described second prior art organic electroluminescent device, however, since a photolithography and etching process is required to form the insulating pattern layer, the manufacturing cost is increased.
In a third prior art organic electroluminescent device, the insulating layer filled between the lower electrodes is formed by the same photoresist pattern layer for forming the lower electrodes, to thereby decrease the manufacturing cost (see: FIG. 6 of JP-A-2000-123978). That is, first, an ITO layer is deposited on a transparent insulating substrate. Then, a photoresist pattern layer is formed on the ITO layer by a photolithography process, and the ITO layer is patterned by an etching process using the photoresist pattern layer as a mask to form lower electrodes. Then, an insulating layer is deposited on the entire surface without removing the photoresist pattern layer, and a so-called lift-off operation is performed on the photoresist pattern layer, so that the photoresist pattern layer and a part of the insulating layer on the photoresist pattern layer are simultaneously removed. As a result, the remainder of the insulating layer is filled as fillers between the lower electrodes.
In the above-described third prior art organic electroluminescent device, however, since the formation of the lower electrodes by an etching process is carried out in a chamber different from a chamber where the formation of the insulating layer is carried out, the manufacturing cost is still high.